In the information processing and telecommunications fields, in order to increase the information transmission amount per channel or the transmission rate, it is important to achieve an optical modulator for performing high speed modulation into optical signals of signals from an LSI circuit in charge of information processing in an optical communication device. Silicon-based optical communication devices functioning at 1310 and 1550 nm fiber-optic communication wavelengths for a variety of systems such as of fiber-to-the-home and local area networks (LANs) are highly promising technologies which enable integration of optical functioning elements and electronic circuits together on a silicon platform by means of CMOS technologies. In recent years, silicon-based passive optical devices such as waveguides, couplers and wavelength filters have been studied very extensively. Important technologies as means for manipulating optical signals for such communication systems include silicon-based active devices such as optical modulators and switches, which also have been attracting much attention. In this respect, optical switches and modulators using a thermo-optic effect of silicon to change the refractive index operate at low speed, and accordingly, their use is limited to that in cases of device speeds corresponding to modulation frequencies not higher than 1 Mb/second. Accordingly, in order to realize a high modulation frequency demanded in a larger number of optical communication systems, optical modulators using an electro-optic effect are required.
Most of the electro-optic modulators proposed to date are devices which use a carrier plasma effect to change the free carrier density in a silicon layer and thereby change the real and imaginary parts of the refractive index, thus changing the phase and intensity of light. Such wide use of the above-mentioned effect is because of the fact that pure silicon does not exhibit a linear electro-optic effect (the Pockels effect) and that a change in its refractive index due to the Franz-Keldysh effect or the Kerr effect is very small. In modulators using free carrier absorption, the output is directly modulated through a change in absorption of light propagating in Si. As a structure using such refractive index change, one employing a Mach-Zehnder interferometer is generally used, where intensity modulated optical signals can be obtained by causing optical phase differences in the two arms to interfere with each other.
Free carrier density in the electro-optical modulators can be varied by injection, accumulation, depletion or inversion of free carriers. Most of such devices having been studied to date have low optical modulation efficiency, and accordingly, for optical phase modulation, require a length on the order of millimeters and an injection current density higher than 1 kA/cm3. In order to realize size reduction, higher integration and also reduction in power consumption, a device structure giving high optical modulation efficiency is required, and if it is achieved, reduction in the optical phase modulation length becomes possible. In case of a large device size, the device becomes susceptible to the influence of temperature distribution over the silicon platform, and it is accordingly anticipated that a change in the refractive index of the silicon layer caused by a thermo-optic effect due to the temperature distribution cancels out the essentially existing electro-optic effect, thus raising a problem.
FIG. 1 shows a typical example of a silicon-based electro-optic phase modulator using a rib waveguide structure formed on an SOI substrate, which is shown in Non-patent Document 1. The structure shown in FIG. 1 corresponds to a PIN diode type modulator, and has a structure where the free carrier density in an intrinsic semiconductor region is changed by applying forward and reverse biases, and the refractive index is accordingly changed using a carrier plasma effect. In the electro-optic phase modulator, the rib waveguide structure is formed using the Si layer on the silicon-on-insulator (SOI) substrate. FIG. 1 is a cross-sectional view of the optical modulator in a plane perpendicular to the propagation direction of light. The optical modulator comprises an oxide layer 12 on the top surface of a silicon substrate 11 as a support substrate. On the top surface of the oxide layer 12, a rib waveguide 14 is formed. The rib waveguide 14 comprises in its central part a protruding portion 15 to become the core, and also comprises slab portions 16 which are present on respective sides of the protruding portion 15 and connected with it. (In the present Description, the protruding portion may be referred to also as a rib portion.) The rib waveguide 14 is an intrinsic semiconductor silicon layer.
Further, on the sides of the respective slab portions 16, a p-type region 17 and an n-type region 18 are respectively formed by p-type or n-type doping processes into the intrinsic semiconductor silicon layer. (A PIN diode structure is thus constructed.) On the top surface of the p-type region 17, a first electrode contact layer 19 is formed, and the first electrode contact layer 19 is then connected with an electrode wiring 21. On the top surface of the n-type region 18, a second electrode contact layer 20 is formed, and the second electrode contact layer 20 is then connected with another electrode wiring 21. In the PIN diode structure, the p-type region 17 and the n-type region 18 may be doped to exhibit a carrier density of about 1020 per unit volume (1 cm3). Then, in a manner to entirely cover the rib waveguide 14, the p-type region 17 and the n-type region 18, an oxide cladding layer 13 to function also as a cladding layer in the waveguide is arranged.
In terms of optical modulation, the optical modulator is connected to a power supply through the first and second electrode contact layers 19 and 20 so as to apply a forward bias to the PIN diode and thereby inject free carriers into the waveguide. When the forward bias is applied, the refractive index of the intrinsic semiconductor silicon layer (that is, the rib waveguide 14) is changed as a result of the increase in free carriers, and phase modulation of light transmitted through the waveguide 14 is thereby performed. Such prior art PIN diode phase modulators generally have an operation speed in the range of 10-50 Mb/second during the forward bias operation. Here, the speed of optical modulation operation is limited by the lifetime of free carriers in the rib waveguide 14 and carrier diffusion in there when the forward bias is removed. In this respect, it is possible to increase the switching speed by introducing impurities into the silicon layer and thereby reducing the carrier lifetime, but it raises a problem in that the introduced impurities deteriorate the optical modulation efficiency.
Further, the most influencing factor on the operation speed is one due to the RC time constant, where the capacitance (C) at a time of forward bias application becomes very large as a result of reduction in the carrier depletion layer width of the PN junction. While, theoretically, high speed operation of the PN junction could be achieved by applying a reverse bias, it requires a relatively high drive voltage or a large device size.
Patent Literature 1 (Japanese translation of PCT international application No. 2006-515082) discloses an example of a silicon-based electro-optical modulator 30, as shown in FIG. 2, in which an SIS (silicon-insulator-silicon) junction is formed.
In the waveguide structure of the optical modulator 30, a p-doped silicon layer 34 and an n-doped silicon layer 38 are stacked across a relatively thin dielectric layer 42. As shown in FIG. 2, the optical modulator 30 comprises a silicon substrate (support substrate) 31, an oxide layer 32, an oxide cladding layer 33 and electrode wirings 41. On the top surface of the oxide layer 32, the relatively thin silicon surface layer 34 doped to have a first type of conductivity is formed. It is assumed that the relatively thin silicon surface layer doped to have a first type of conductivity (for example, doped with p-type dopants) is referred to as a body region 34. Above the top surface of the body region 34, a gate region 38 is formed in a manner to at least partly overlap with the body region 34. The gate region 38 is formed of a relatively thin silicon region doped to have a second type of conductivity (for example, doped with n-type dopants). Between the body region 34 and the gate region 38, a thin gate dielectric 42 is interposed.
In an end portion of the body region 34 (the right end portion in FIG. 2), a heavily doped region 35 is formed by a high concentration doping process, then a first electrode contact layer 36 is formed on the top surface of the heavily doped region 35, and the first electrode contact layer 36 is connected to an electrode wiring 37. In an end portion of the gate region 38 (the left end portion in FIG. 2), a heavily doped region 39 is formed by a high concentration doping process, then a second electrode contact layer 40 is formed on the top surface of the heavily doped region 39, and the second electrode contact layer 40 is connected to an electrode wiring 41.
In the configuration described above, doping processes are applied to the gate region 38 and the body region 34, where the resultant doped portions are defined such that the carrier density change is controlled there by an external signal voltage. Then, it is ideally desirable to make an optical signal electric field coincide with the region where the carrier density is externally and dynamically controlled, in which situation optical phase modulation can be performed by accumulating, depleting or inverting free carriers on each side of the gate dielectric layer 42. However, there practically is a problem in that the region where the carrier density dynamically changes is as thin as about a few tens of nanometers, which results in a problem in that an optical modulation length on the order of millimeters is required, the electro-optical modulator accordingly becomes large in size, and high speed operation consequently becomes difficult.